The development of mouse CNS will be studied by light microscopy, scanning EM, and transmission EM of organotypic cultures; by neuroanatomic analysis of neurological mutant diseases; and by culture experiments on mutant tissues. Understanding of normal neuroanatomical development in cultures permits their use to distinguish extrinsic from intrinsic and genetically programmed from inductive influences in normal development and in the abnormalities of neurological mutant or neurotoxic diseases. Specific investigations will be: correlated comparative examination of normal and myelin-deficient mutant cultures: correlated morphological study of growth of nerve fibers in Xenopus laevis tadpoles, as a control on the verisimilitude of the mouse cultures: culture of tissues from myelin-deficient and cerebellar mutants not previously studied in vitro: morphologic studies on organotypic explant cultures versus dissociated and reaggregated cell cultures of hippocampal formation: and investigation of abnormal brain anatomy in circling mutant mice, which have severe inner ear disease but have not been investigated for CNS neuropathology.